1. Field of the Invention
The present invention generally relates to high efficiency RF (Radio Frequency) power amplifiers, and more particularly to a high efficiency RF power amplifier having an improved circuit configuration which creates a short circuit impedance at the second harmonic of the fundamental operating frequency.
2. Description of the Prior Art
Recently, portable telephone sets or mobile radio communication devices have been practically used. These devices use a battery as a power source. A transmission power amplifier of the devices consumes approximately 75% of power supplied by the battery. Thus, it is necessary to improve the efficiency of the transmission power amplifier.
A conventional high efficiency power amplifier used in the microwave range uses a GaAs field effect transistor, as disclosed in U.S. Pat. No. 4,717,884, the disclosure of which is hereby incorporated by reference.
FIG. 1 is a circuit diagram of the high efficiency RF power amplifier disclosed in the above patent. The amplifier shown in FIG. 1 has a GaAs field effect transistor 10, which has a grounded source, a gate receiving an RF input signal, and a drain. The amplifier functions as a class F amplifier capable of efficiently amplifying input power.
At the third harmonic of the fundamental operating frequency, a parallel resonance circuit which comprises a lead inductance L1 of a bonding wire, a parasitic capacitance C1 of the GaAs FET 10, and an external inductor 11 is created, so that the drain of the GaAs FET 10 is opened (in other words, a high-impedance circuit is connected to the drain of the GaAs FET 10). In this case, a stub 12 formed of, for example, a micro-strip line, has a length of .lambda..sub.3 /4 where .lambda..sub.3 is the wavelength of the third harmonic. At the second harmonic of the fundamental operating frequency, a series resonance circuit composed of the capacitance of the stub 12, the lead inductance L1 and the external inductor 11 is created, so that the drain of the GaAs FET 10 is substantially shortcircuited (in other words, a low-impedance circuit is connected to the drain of the GaAs FET 10). It will be noted that the amplifier shown in FIG. 1 is designed taking into account the lead inductance L1 and the parasitic capacitance C1.
A choke coil 13 is coupled to the drain of the GaAs FET 10 via the bonding wire having the lead inductance L1. A drain bias voltage is applied to the drain of the GaAs FET 10 via the choke coil 13 and the lead inductance L1. It is necessary for the choke coil 13 to have a high impedance in the frequency range between the fundamental operating frequency and the third harmonic thereof. If this requirement is not satisfied, the above-mentioned short and open circuits will not be created, so that the second and third harmonics cannot be removed effectively. In order to meet the above requirement, the choke coil 13 must be formed of a lumped-constant circuit element, such as a coil obtained by turning a wire several times. In other words, the choke coil 13 cannot be formed of a distributed-constant circuit, such as a strip line. When the lumped-constant circuit element is used for forming the drain bias circuit, it is difficult to provide a compact, less expensive RF power amplifier.